The present invention relates to a method for treating boil-off gas in an LNG carrier having a reliquefaction apparatus, and more particularly, to a method and an apparatus for treating boil-off gas which can prevent waste of boil-off gas and save energy by storing in an LNG storage tank, instead of discharging and burning, surplus boil-off gas which has not been returned to the LNG storage tank through a reliquefaction plant among the total amount of boil-off gas generated in the LNG storage tank.
Generally, natural Gas (NG) is turned into a liquid (also called liquefied natural gas or LNG) in a liquefaction plant, transported over long distances by an LNG carrier, and regasified by passing a floating storage and regasification unit (FSRU) or an unloading terminal on land to be supplied to consumers.
As liquefaction of natural gas occurs at a cryogenic temperature of approximately −163° C. at ambient pressure, LNG is likely to be vaporized even when the temperature of the LNG is slightly higher than −163° C. at ambient pressure. Though an LNG storage tank of an LNG carrier is thermally insulated, as heat is continually transmitted from the outside to the LNG in the LNG storage tank, the LNG is continually vaporized and boil-off gas (BOG) is generated in the LNG storage tank during the transportation of LNG by the LNG carrier.
If boil-off gas is generated in an LNG storage tank as described above, the pressure of the LNG storage tank is increased and becomes dangerous.
Conventionally, if the pressure of an LNG storage tank is increased beyond a set pressure, boil-off gas was discharged to the outside of the LNG storage tank and used as a fuel for propulsion of an LNG carrier, so as to maintain the pressure of the LNG storage tank at a safe level. However, a steam turbine propulsion system driven by the steam generated in a boiler by burning the boil-off gas generated in an LNG storage tank has a problem of low propulsion efficiency.
Also, a dual fuel diesel electric propulsion system, which uses the boil-off gas generated in an LNG storage tank as a fuel for a diesel engine after compressing the boil-off gas, has higher propulsion efficiency than the steam turbine propulsion system, but has difficulty in maintenance due to complicated integration of a medium-speed diesel engine and an electric propulsion unit in the system. In addition, this system, which must supply boil-off gas as a fuel, is forced to employ a gas compression method which requires higher installation and operational costs than a liquid compression method.
Further, such a conventional method using boil-off gas as a fuel for propulsion fails to achieve the efficiency of a two-stroke slow-speed diesel engine, which is used in ordinary ships.
Furthermore, the conventional method has such another problem that, in case the amount of boil-off gas generated in an LNG storage tank exceeds the capacity of a propulsion system, additional equipment such as a gas combustion unit is needed to treat surplus boil-off gas.
On the other hand, there is another method of maintaining a pressure of an LNG storage tank at a safe level. If the pressure of the LNG storage tank is increased beyond a set pressure, boil-off gas is discharged to the outside of the LNG storage tank and reliquefied in a reliquefaction plant and then returned to the LNG storage tank.
FIG. 1 shows a conceptual diagram for explaining a method for treating boil-off gas in an LNG carrier having a reliquefaction plant.
As shown in FIG. 1, the LNG carrier having a reliquefaction plant comprises an LNG storage tank (1) for storing LNG therein, a boil-off gas compression unit (110) for compressing boil-off gas generated in the LNG storage tank (1), a condenser (120) for condensing the compressed boil-off gas by exchanging heat with a refrigerant, and a refrigerant system (130) for providing cold heat for condensing boil-off gas in the condenser (120). Here, the boil-off gas compression unit (110), the condenser (120) and the refrigerant system (130) constitute the reliquefaction plant.
Though the reliquefaction plant is provided on the LNG carrier, a treatment capacity of the reliquefaction plant is limited, and in case an amount of boil-off gas greater than the treatment capacity of the reliquefaction plant is generated, surplus boil-off gas must be burned and wasted. To burn surplus boil-off gas, a conventional LNG carrier has a gas combustion unit (103), and the surplus boil-off gas is heated in a gas heater (105) to an appropriate temperature and then supplied to the gas combustion unit (103) to be burned and wasted.
FIG. 2 illustrates a graph showing changes over time in an internal pressure of an LNG storage tank and in an amount of boil-off gas generated in the LNG storage tank according to a conventional boil-off gas treating method.
As illustrated in FIG. 2, in case a constant internal pressure of the LNG storage tank (1) is maintained at approximately 106 kPa, a large amount of boil-off gas is discharged to the outside of the LNG storage tank (1) for 3 to 4 days at the beginning of a loaded voyage of the LNG carrier, and an amount of boil-off gas discharged becomes stable (approximately 5,643 kg/hr in FIG. 2) after 3 to 4 days from the beginning of the loaded voyage. Conventionally, a treatment capacity of a reliquefaction plant was determined based on this stable amount of boil-off gas discharged.
Since a treatment capacity of a reliquefaction plant is limited, surplus boil-off gas beyond a treatment capacity of a reliquefaction plant is generated for 3 to 4 days at the beginning of a loaded voyage of an LNG carrier. Such surplus boil-off gas, as stated above, is all burned and wasted. Accordingly, the prior art has a problem that large quantities of surplus boil-off gas, which amount to 55 tons (see oblique lines in FIG. 2), are burned and wasted.
In a case of an LNG carrier having a capacity of 150,000 m3, the quantity of boil-off gas burnt as described above amounts to 1500 to 2000 tons per year, which cost about 700,000 USD. Further, burning of boil-off gas raises a problem of environmental pollution.
In addition, the prior art has such other problems that since a reliquefaction plant and a gas combustion unit (103) should be operated together at the beginning of a loaded voyage of an LNG carrier, additional equipment such as a gas combustion unit (103) or a gas heater (105) is needed for treating the surplus boil-off gas, and that a large amount of energy is consumed due to operation of the gas combustion unit (103).
Korean Patent Laid-Open Publication Nos. KR 2001-0014021, KR 2001-0014033, KR 2001-0083920, KR 2001-0082235, and KR 2004-0015294 disclose techniques of suppressing the generation of boil-off gas in an LNG storage tank by maintaining the pressure of the boil-off gas in the LNG storage tank at a high pressure of approximately 200 bar (gauge pressure) without installing a thermal insulation wall in the LNG storage tank. However, this LNG storage tank must have a significantly high thickness to store boil-off gas having a high pressure of approximately 200 bar, and consequently it has problems of increasing manufacturing costs and requiring additional equipment such as a high-pressure pump, to maintain the pressure of boil-off gas at approximately 200 bar.
As stated above, a method for treating boil-off gas in an LNG carrier according to the prior art, which maintains an internal pressure of an LNG storage tank at a constant level and allows generation of boil-off gas during transportation of a cryogenic liquid, has a problem of consuming a large amount of boil-off gas or having to install additional equipment such as a reliquefaction plant and a gas combustion unit.
In addition, unlike a case of transporting a cryogenic liquid at a low atmospheric pressure, a method of transporting a cryogenic liquid using a storage tank, such as a pressure tank, which can withstand a high pressure at a somewhat high temperature, does not need to treat or waste boil-off gas, but has problems that the size of the tank is limited and that high manufacturing costs are required.